Evaporated solderable multilayer contact for silicon semiconductor

ABSTRACT

An evaporated multilayer solderable low resistance contact for N-type and P-type regions on a semiconductor body comprising an aluminum layer directly on the semiconductor body, an evaporated manganese layer on the aluminum layer and an evaporated nickel layer on the manganese layer.

RELATED PATENT APPLICATION

This is a division of U.S. Patent Application Ser. No. 605,979, filedAug. 20, 1975, now U.S. Pat. No. 3,990,094.

BACKGROUND OF THE INVENTION

This invention relates to ohmic contacts and semiconductor bodies andmore particularly to an improved multilayer low resistance solderablecontact that can be used on both N-type silicon and P-type silicon.

In the past, nickel layers have been used as single layer solderableohmic contacts directly on N-type silicon. In such contacts, the nickellayer is applied by electroless deposition from an aqueous solutioncontaining nickel sulfate and sodium hypophosphite. The plated siliconbody is heated after the nickel is deposited. After heating at amoderate temperature, the nickel layer has a low contact resistance onN-type silicon. This is due to a significant phosphorus concentration inthe nickel layer. However, the phosphorus concentration that reducescontact resistance on N-type silicon, increases it on P-type silicon.Hence, for lowest resistance solderable ohmic contacts on P-typesilicon, other aproaches have been used.

Excellent layer resistance contacts are regularly made to P-type siliconwith a specially microalloyed aluminum layer. However, aluminum is notreadily solderable. It is generally known to coat aluminum with one ormore layers of another metal, to provide an outer layer that issolderable. Various metals and deposition techniques can be used inmaking semiconductor devices, vacuum deposition is frequently used.Coatings of pure nickel can be conveniently applied to aluminum by avacuum deposition. Pure nickel provides a highly solderable surface anddoes not introduce undesirable impurities to the semiconductor surface.However, the adhesion of pure nickel to aluminum is unsatisfactory. Itis not as strong as the aluminum-silicon bond, or the nickel-solderbond.

We have found that it is as difficult to get pure nickel tosatisfactorily adhere to aluminum as it is to get solder to do so. Forexample, when a silicon element having an aluminum-pure nickelmultilayer contact is soldered to a supporting substrate and subjectedto bending stresses, the nickel separates from the aluminum to produceelectrode failure.

In our U.S. Pat. No. 3,886,585 entitled "Solderable Multilayer Contactfor Silicon Semiconductor" we disclosed that a vacuum depositedmanganese-nickel alloy adhered better than pure nickel to an aluminumsurface. Moreover, the manganese-nickel alloy did not introduceundesirable impurities to the semiconductor surface, increase the numberof processing steps, or reduce solderability.

Such an alloy is readily deposited by sputtering, and by moresophisticated electrical resistance evaporation techniques. However, itis not so readily deposited, particularly under commercial productionconditions, with a multiple source electron beam evaporation apparatusor the more rudimentary filament type electrical resistance heatedevaporation apparatus. Vapor pressure difference between the nickel andmanganese can produce control problems with the latter apparatus thatare objectionable for commercial production operations. A givencommercial production facility may have a substantial capital investmentin this latter vacuum deposition apparatus. If so, it may not beeconomically prudent to convert to sputtering apparatus or to a moresophisticated electrical resistance heated evaporation apparatus. Insuch instance there may be a reluctance to use our prior invention inthat given production facility.

On the other hand, we have found that the manganese and nickel need notbe simultaneously evaporated as an alloy. Extremely adherent solderablecontacts can be obtained by successively evaporating the manganese andnickel in a single evacuation step. High yields can be consistentlyobtained under commercial production conditions with the multiple sourceelectron beam heated evaporation apparatus and with the more rudimentaryfilament type resistance heated evaporation apparatus referred toearlier. Also, sputtering apparatus or more sophisticated electricalresistance heated evaporation apparatus can be used, but are preferablyused with our invention disclosed in the aforementioned U.S. Pat. No.3,886,585.

OBJECTS AND SUMMARY OF THE INVENTION

The principal objects of this invention are to provide an improvedevaporated multilayer solderable contact on silicon and method ofproducing such a contact that is particularly useful with selectedcommercial production vacuum deposition apparatus. These and otherobjects of the invention are obtained with an aluminum layer on silicon,and successively evaporated layers of manganese and nickel. Themanganese and nickel are preferably evaporated in a single evacuationstep.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE in the drawing diagrammatically shows a terminal leadsoldered to a multilayered electrode made in accordance with theinvention

DESCRIPTION OF THE PREFERRED EMBODIMENT

The contact of this invention can be used to ohmically attach asemiconductor die to a supporting substrate, or to ohmically attach aterminal lead to the die. The drawing illustrates the latter, and servesas one specific example of the invention. The layers shown are not drawnto scale, to better illustrate the novel multilayer contact involved.The multilayer contact is formed on a P-type portion 10 of a siliconsemiconductor device. This portion, for example, can be the collectorregion of a PNP transistor or the base region of an NPN transistor. Thefilm 12 of aluminum is on the surface 14 of portion 10, and microalloyedthereto. This produces a low resistance electrical connection, an ohmiccontact, between the aluminum layer 12 and P-type silicon portion 10.Aluminum film 12 can be about 3000 - 15000 angstroms thick. Itsthickness is no more critical in the electrode of this invention than itis in the usual single layer aluminum ohmic contacts on N-type andP-type silicon.

A 1000 angstroms thick film 16 consisting essentially of manganese is onthe aluminum layer 12. We prefer the manganese be substantially pure,except for electronically neutral metals that may enhance adhesion tothe microalloyed aluminum and to the nickel. By electronically neutralmetals we mean metals that are not conductivity determining impuritiesin silicon. Film 16 has an average thickness of at least 800 angstromsto obtain a substantially continuous layer. Thinner layers may be usefulbut are difficult to measure and their deposition is difficult tocontrol, particularly with respect to thickness uniformity. We havefound no upper limit on manganese thickness that can be used. However,using a film thickness that is any greater than necessary to insurecomplete aluminum coverage with the apparatus used appears to provide noincreased benefits. We prefer to use a thickness below about 2500angstroms. Essentially, using a significantly thicker manganese filmmerely increases evaporation time and expense. Of course, an increase inevaporation time slows production rate through a given evaporationapparatus.

A film 18 consisting essentially of nickel is on the manganese film 16.For commercial production operations we prefer that nickel film 18 besubstantially pure, except for electronically neutral metal contentwhich may enhance solderability and/or adhesion of the nickel layer tothe evaporated manganese.

The average thickness of nickel film 18 can be varied over broad limits.However, sufficient nickel must be present to provide a continuouscoating on the manganese film and allow good solderability. An averagethickness greater than about 3000 angstroms is usually necessary toinsure that a nickel coating will not only be continuous initially butremain so after solutioning effects of the solder. Average nickelthicknesses greater than about 5000 angstroms offer no significantadvantage in reliability or yields. Such thicker layers increase costand slow down output of given evaporation apparatus, similar toincreased thickness of manganese film 16.

A Kovar terminal lead 20 is attached to the nickel film 18 by means ofan interjacent solder layer 22. Solder layer 22 can be of any suitablesolder, such as 90% by weight lead and 10% by weight tin.

In essence, our sequential manganese-nickel multilayer coating providesthe same two purposes as the single manganese-nickel alloy layer in ouraforementioned U.S. Pat. No. 3,886,585. It provides an adherent coatingon a microalloyed aluminum film, and also provides a coating that has asolderable surface. Solder is not the only metal that will readilyadhere to nickel film 18. Thus, the nickel layer need not be the last orouter layer of our solderable electrode. One or more additional vacuumdeposited layers of metal could be used over our nickel film 18, so longas the last layer applied provides a solderable surface. Additionallayers of pure nickel, silver or gold might be used. On the other hand,since our nickel film 18 is itself quite solderable, we prefer to useonly the three layers, aluminum 12, manganese 16 and nickel 18. Anyadditional layers would preferably be applied by evaporation rather thansputtering, and be applied in a single evaporation step using a multiplesource evaporation apparatus.

As in our aforementioned U.S. Pat. No. 3,886,585 our electrode is ofspecial interest in providing an adherent low resistance solderablecontact for P-type silicon, because no such contact is available forP-type silicon. On the other hand, it works equally well on N-typesilicon. Silicon devices usually have both N-type and P-type regions. Inthis invention the same metallization system can be used for goodsolderable contacts on both conductivity type regions. Our multilayeredelectrode can be used on both conductivity type regions because theinitial layer of our contact is a microalloyed aluminum film. It issolderable because the outer layer is of a solderable metal. Thedifficulty with such a contact is getting adequate adhesion between thealuminum and the subsequently applied metal layers. It is normally theweakest adhesion link in the electrode metallization system.

Our manganese and nickel films can be successively vacuum depositeddirectly onto a new or old microalloyed film. As previously pointed out,this particular metallization system is particularly useful inconnection with less sophisticated resistance heated filament type orelectron beam heated evaporation apparatus. Such apparatus has been usedextensively in commercial production operations in the semiconductorindustry. Only the usual degree of care need be used to consistentlyobtain extremely highly adherent solderable contacts under commercialproduction conditions. It is recognized that deposition of two layerstakes more evaporation time than that of a single layer, such as in ourprior invention. However this evaporation time can be reduced if boththe manganese and nickel layers are successively deposited in a multiplesource evaporation apparatus in a single evacuation step. This type ofapparatus is already widely used in commercial production operations.

To make an adherent solderable multilayer electrode in accordance withthis invention, a clean silicon substrate is placed in a vacuumevaporation chamber, and the chamber pumped down to a pressure of about1 × 10⁻ ⁶ Torr. The silicon substrate is preferably moderately warm toenhance adhesion of the aluminum to the silicon. While any substratetemperature up to about 300° C. can be used, temperatures in excess of150° C. provide best results, and we prefer 200° C. Aluminum is thenevaporated from a tungsten electrical resistance heater onto the warmsilicon substrate until a 10,000 angstrom thick layer of aluminum isdeposited on the substrate. The substrate is then removed from theevaporation chamber, and the aluminum layer microalloyed to thesubstrate surface. For microalloying, the aluminum coated substrate isplaced for 3 - 5 minutes in a furnace tube that is heated to atemperature of about 500° - 575° C. and has an argon atmosphere. Thesubstrate is then immediately removed from the furnace tube and allowedto cool in room air to room temperature. After cooling to roomtemperature, the substrate is placed back in the vacuum evaporationchamber. The vacuum evaporation chamber is again evacuated to a pressureof 1 × 10⁻ ⁶ Torr. As with the aluminum layer, the silicon substrate ispreferably moderately warmed during the vacuum deposition in thischamber. A substrate temperature of about 200° - 260° C. is preferred. A1000 angstrom thick manganese film is then evaporated onto themicroalloyed aluminum layer from a tungsten resistance heater. If theevaporation apparatus is of the multiple source type, a second source isthen activated and a nickel film is then evaporated onto the manganesefilm. The substrate is then cooled to less than 100° C., the vacuumchamber brought to atmospheric pressure, the the substrate removed fromthe chamber.

If the apparatus is not of the multiple source evaporation type, thesubstrate is cooled and the vacuum chamber opened as mentioned in thepreceding paragraph. However, the substrate is retained in position inthe chamber and the tungsten electrical resistance heater is loaded withnickel. The chamber is then closed and evacuated again to a pressure of1 × 10⁻ ⁶ Torr. The substrate is heated again to a temperature of about200° - 260° C., and the nickel evaporated onto the manganese film. Thesubstrate is then cooled again to a temperature of less than 100° C.,the chamber brought up to atmospheric pressure, opened, and thesubstrate removed from the vacuum evaporation chamber. A terminal lead20 can then be soldered to the nickel film 18 in the usual manner.

It should also be recognized that all the layers of our multilayercontact, including the aluminum layer, can be produced in a singleevacuation step. The substrate need not be removed from the vacuumevaporation chamber for microalloying. However, multiple source vacuumdeposition equipment would be required. Moreover, it must have thecapability of heating the silicon substrate to a microalloyingtemperature, and then quickly cooling it back to the vacuum depositiontemperature. In such event the substrate is placed in the vacuumdeposition chamber, the chamber pumped down to a pressure of about 1 ×10⁻ ⁶ Torr and the substrate concurrently moderately heated. A 10,000angstrom thick aluminum layer is then deposited onto the substrate, thesubstrate heated to about 500° - 575° C. for approximately 3 - 5minutes, and then quickly cooled to about 200° - 260° C. As is known, ifquick cooling is not provided, the contact will still be of lowresistance on P-type silicon but not on N-type silicon. After the quickcooling the manganese and nickel films can then be successivelydeposited onto the microalloyed aluminum layer in the mannerhereinbefore described. In this connection it should be mentioned thatdifferent metals can be evaporated in a single evaporation chamberusually by simply rotating another source or target into position, orunshielding another source, without opening the vacuum evaporationchamber. An aging bake could be added but, because of the time involved,the slices would have to be removed from the system for this step. Theaging bake step is used after the aluminum layer is microalloyed to thesilicon substrate surface. In the aging bake step, the silicon substratehaving the microalloyed aluminum layer on it is heated for 4 hours atabout 400° C. in dry air. The aging bake step after microalloying is aknown and accepted procedure preferred by some and omitted by others. Itis no more important to this invention than it is in other microalloyedaluminum contacts.

We claim:
 1. A method of forming an improved solderable multilayerelectrode on a silicon body comprising the steps of:vacuum depositing analuminum layer at least about 3000 angstroms thick onto a surface of asilicon body, heating said silicon body and said aluminum layer toshallowly alloy said aluminum layer with the silicon of said body attheir interface and reduce electrical resistance between said layer andsaid body, thereafter evaporating onto said alloyed aluminum layer acontinuous film of manganese having an average thickness of at leastabout 800 angstroms, and evaporating nickel onto said manganese film toform a nickel layer at least about 3000 angstroms thick.
 2. A method offorming a more adherent low resistance solderable multilayer electrodeon a silicon surface comprising the steps of:vacuum depositing analuminum layer about 3000- 15,000 angstroms thick onto a siliconsurface, microalloying said aluminum layer to said silicon surface,wherein the electrical resistance therebetween is reduced and thealuminum layer is more intimately bonded to said silicon surface,thereafter evaporating manganese onto said microalloyed aluminum layerto form a manganese layer about 800- 2500 angstroms thick, and withoutexposing said manganese layer to air, evaporating nickel onto saidmanganese layer to form a nickel layer thickness of about 3000- 5000angstroms.